Go to "border selection" (Default key 3), select one edge of the hole, all other should be selected automatically) and then click on "cap" in the Edit Rollout. This will fill the hole with a single n-gon. You can then go to vertex mode (default: 1) select the vertex and use connect to create 4-sided Polygons. Another…
Here's an extra technique that produces nice quads on certain amounts of segments that wasn't mentioned yet; 1) use the Border selection mode to select the open edge 2) deselect two opposing edges 3) hit the "Bridge" tool